Whilst I am in agreeance with all advise, I also happen to disagree with it as well!!!
I too have been fixing domestic fences as a business for many years.
And sloping the top of the post is a great start, but is hard on farm pence posts to carry out.
I have installed house stump ant caps on many farm posts.
Posts will certainly rot in the area of the soil that changes from dry to wet over time, continually.
But often decks are built with no post in the soil.
I have seen many posts in poor condition because of water entry at the top.
In fact I suggested to somebody last week they
should think about capping the post during the restoration they were undertaking.
Anything that does not blow away will work.
\ But, why is copper too expensive, compared with the cost of post removal?